The general rule of thumb is a high quality 500 - 550 watt power supply can easily power just about an single video card made.
The 9800GTX cards do not use very much power. I checked several resources. There were different numbers for different testing methods but it looks like the card will probably use a maximum of about 125 watts at load.
The rest of your system does not use very much power either.
Here is a link to a technical review of the Antec Neopower 550 watt power supply: